Firstly I'd make sure that you really need XA for your solution. XA is used for managing a global txn across multiple resources and it is therefore quite heavyweight. Sometimes having a single transaction for each resource is good enough (that'll depend on your business requirements)
Configuring XA in Jboss can be quite complex (depending on whether you are involving Database, JMS Messaging, JCA connectors etc). I'm assuming you want to do this for your 2 database model.
1.) You need to make sure your databases are XA capable and have that switched on, Sybase for example require a separate license for XA capability.
2.) You need to set up a whole bunch of configuration in jboss, I recommend starting here:
http://wiki.jboss.org/wiki/ConfigDataSources I'm also going to add this from Theserverside that I thought was an excellent explanation of the concepts:
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An XA transaction, in the most general terms, is a "global transaction" that may span multiple resources. A non-XA transaction always involves just one resource.
An XA transaction involves a coordinating transaction manager, with one or more databases (or other resources, like JMS) all involved in a single global transaction. Non-XA transactions have no transaction coordinator, and a single resource is doing all its transaction work itself (this is sometimes called local transactions).
XA transactions come from the X/Open group specification on distributed, global transactions. JTA includes the X/Open XA spec, in modified form.
Most stuff in the world is non-XA - a
Servlet or
EJB or plain old
JDBC in a
Java application talking to a single database. XA gets involved when you want to work with multiple resources - 2 or more databases, a database and a JMS connection, all of those plus maybe a JCA resource - all in a single transaction. In this scenario, you'll have an app server like Websphere or Weblogic or JBoss acting as the Transaction Manager, and your various resources (Oracle, Sybase, IBM MQ JMS, SAP, whatever) acting as transaction resources. Your code can then update/delete/publish/whatever across the many resources. When you say "commit", the results are commited across all of the resources. When you say "rollback", _everything_ is rolled back across all resources.
The Transaction Manager coordinates all of this through a protocol called Two Phase Commit (2PC). This protocol also has to be supported by the individual resources.
In terms of datasources, an XA datasource is a data source that can participate in an XA global transaction. A non-XA datasource generally can't participate in a global transaction (sort of - some people implement what's called a "last participant" optimization that can let you do this for exactly one non-XA item).
For more details - see the JTA pages on java.sun.com. Look at the XAResource and Xid interfaces in JTA. See the X/Open XA Distributed Transaction specification. Do a google source on "Java JTA XA transaction".
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